1. Field of the Invention
The present invention relates to a device for controlling air-fuel (A/F) ratio of an internal combustion engine and, more particularly, to an air-fuel ratio controller which stops an oxygen pump current and a power supply to a heater when the engine stops.
2. Description of the Prior Art
According to the art proposed by Japanese Utility Model Application Laid-Open No. 62-18659 (1987), using a wide-range air-fuel ratio sensor composed of an oxygen sensor for generating electromotive force according to the difference of oxygen concentration between the atmosphere and exhaust gas from an internal combustion engine and an oxygen pump for allowing pump current to flow in order to take oxygen into and out of exhaust gas for comparison with the atmosphere, the pump current is controlled in order that voltage outputted from the oxygen sensor can be a predetermined value. The proposed air-fuel ratio sensor detects the air-fuel ratio of the engine according to the magnitude of pump current. The air-fuel ratio sensor proposed by the art can continuously measure the air-fuel ratio from rich to lean degree. Conventionally, any A/F ratio controller detects air-fuel ratio by means of pump current by allowing pump current to flow in order that the voltage outputted from the oxygen sensor can be the predetermined value as mentioned above. However, when the engine stops, since an excessive quality of oxygen exists, greater a quantity of pump current must be generated. If the wide-range A/F ratio sensor is operated in presence of the above condition, stabilized zirconia used for composing the oxygen pump or the elements of electrodes used for extracting pump current will degrade themselves to eventually shorten the service life of the wide-range A/F ratio sensor.
In order to activate both the oxygen sensor and the oxygen pump, these must be heated to a high temperature. To achieve this, a heater is provided. Nevertheless, since the heater has substantial current capacity, if the key switch remains ON while the engine stops, its battery will continuously discharge voltage to shorten its own service life.